Distorted planar mapping

I'm having a strange issue with a project I'm working on and would appreciate any advice from the Creative Cow community.

I'm trying to create a series of posters for a shop window I'm making. I don't want to create a separate material for each of them so I've created poly planes of roughly the right dimensions for each bit of artwork and I've planar mapped each of them and arranged them in the UV Editor so they're not overlapping. I've exported a UV snapshot and created a texture in Photoshop with the artwork for each poster in the corresponding position. When I apply the texture all the images appear as I would expect, except that they all show very pronounced lens-type distortion. The closer to the edges the worse the distortion.